The present invention relates in general to the detection of special nuclear materials (“SNM”) in suspect containers. In particular, the present invention uses high-energy gamma rays emitted from fission products or fragments to identify SNM (i.e., 235U and 239Pu) in cargo containers and other potential sites. Special nuclear material (SNM) is defined by Title I of the Atomic Energy Act of 1954 as plutonium, uranium-233, or uranium enriched in the isotopes uranium-233 or uranium-235. The definition includes any other material which the Nuclear Regulatory Commission determines to be special nuclear material, but does not include source material. The Nuclear Regulatory Commission (“NRC”) has not declared any other material as SNM. SNM is only mildly radioactive, but it includes some fissile material, uranium-233, uranium-235, and plutonium-239, that, in concentrated form, can be the primary ingredients of nuclear explosives. These materials, in quantities greater than formula quantities, are defined as “strategic special nuclear material” (SSNM). The uranium-235 content of low-enriched uranium can be concentrated (i.e., enriched) to make highly enriched uranium, the primary ingredient of a nuclear weapon.
Since Sep. 11, 2001, an increased urgency has been associated with the development of new and improved means for the detection and prevention of the clandestine transport of nuclear weapons materials and other materials for producing weapons of mass destruction. A particularly difficult problem is posed by highly-enriched uranium (e.g., 235U) and plutonium (e.g., 239Pu) that might be hidden in large sea-going cargo containers, which may be filled with masses approaching 27 MT and which might represent areal densities of more than 50 g cm−2 through which an identifying signal must penetrate to reach a detector. Passive detection methods (e.g., see “Passive Nondestructive Assay of Nuclear Materials,” edited by D. Reilly, N. Ensslin, and H. Smith, Jr., NUREG/CR-5550, LA-UR-90-732 (1991)) based on measurements of neutrons and/or photons are either inapplicable or impractical in many such cases. Traditional methods of radiography are unlikely to provide a unique signature of highly-enriched 235U and 239Pu. Active interrogation with neutrons or high-energy photons in a variety of forms (e.g., see “Ionizing Radiation Imaging Technologies for Homeland Security,” D. J. Strom and J. Callerame, Proceedings of the 36th Midyear Topical Meeting, Health Physics Society, Jan. 26-29, 2003, San Antonio, Tex., and “A Review of Neutron Based Non-Intrusive Inspection Technologies,” T. Gozani, Conference on Technology for Preventing Terrorism, Hoover Institution, Mar. 12-13, 2002, Stanford University, Stanford, Calif.) currently depends upon the observation of β-delayed neutrons following induced fission to provide a unique signature for 235U and 239Pu. However, the shielding provided by a thick hydrogenous cargo can be so large that this method will fail or will have very low detection sensitivity.
In addition, considering that millions of cargo and other containers enter the United States each year, and considering that SNM might be hidden in some of these containers, in order to prevent the entry of any hidden SNM into the United States, a detection method needs to be effective without having to open and unload the containers. Furthermore, not only does a detection system need to be non-invasive, it must be able to perform its detection function in as short a time as possible, so as to not overly burden the flow of goods into the U.S. via these containers.
There is therefore a need for a system and a method of detecting special nuclear materials (“SNM”) in suspect containers that does not suffer from the above described shortcomings.